This invention relates in general to a material handling system for use in a dip coating process, and, more specifically, to a dip coating process system for use in coating drums of different lengths.
Electrostatographic imaging systems, which are well known, involve the formation and development of electrostatic latent images on an imaging surface of an electrostatographic or photoreceptor. Electrostatographic imaging members are well known and commonly comprise, for example, a hollow cylindrical drum substrate coated with one or more coatings. Typical coatings include a charge generating layer and a charge transport layer. An optional blocking layer is often applied to the drum substrate. Such multi-layered photoconductive devices comprising a photogenerating or charge generating layer and a charge transport layer deposited on a conductive substrate have been disclosed in the art, as for example, in U.S. Pat. No. 4,265,990, the entire disclosure of this patent being incorporated herein by reference. These photoreceptor drums are usually fabricated by dip coating.
Dip coating of hollow cylindrical members such as, for example, a pipe for forming a photoconductive drum has conventionally been carried out by sequentially transporting, via automated conveyors, a plurality of drums into independent coating booths separated by driers and cooling zones. In a typical system, transport pallets containing as many as four substrate pipes are received from a final pipe cleaning station along an assembly line and sequentially transferred into three coating booths, one for each of the following coating layers: an undercoating layer (UCL); a charge generating layer (CGL); and a charge transport layer (CTL). Three drying/cooling zones follow each coating booth and, finally, a load/unload robot is utilized, where each coated drum is removed from the assembly line. Each of the three coating booths contains an indexing mechanism for rotating the pipes through a series of stations for applying the respective coating material, each coating booth containing a pallet/pipe transfer station, a dip coating station, a flash-off station, and a bottom edge wipe station.
The operation of the system described above proceeds in the following manner. Initially, two transport pallets of four pipes each are transported along a conveyor to the pallet/pipe transfer station where the pipes (eight at a time) are raised up from the transport pallets for removal and transfer to the indexing machine. The indexing machine grasps each pipe from the inside diameter by means of a chucking device for carrying the pipes to each station in the particular coating booth. After receiving the pipes at the pallet/pipe transfer station, the indexer rotates sequentially in 90.degree. increments to deliver the pipes to each processing station. The pipes are first delivered to the dip coating station where a plurality of individual dip tanks are raised around each pipe for receiving each pipe to individually dip coat each pipe. In this manner, the dip tanks are raised around the pipes, come to rest with the pipes therein, and finally lowered in accordance with a specific time and velocity profile for providing a coating having a predetermined thickness for the particular layer being applied to the pipe.
After the pipes have been dipped for a predetermined amount of time, the dip tanks are lowered away from the pipes and the indexing mechanism rotates to transport the pipes to a flash-off station. At this station, solvent vapor from the coating formula is allowed to dissipate or "flash-off". After a sufficient flash-off time, the indexer once again rotates to a bottom edge wipe station. At this station, a boundary area of approximately 11 mm along the bottom rim of the coated pipe is cleaned off by means of a combination solvent and brush contact to remove the coating layer deposited thereon. This bottom edge wipe step is necessitated by the fact that the bottom edge portion of the drum is used as an electrical contact point when placed in the electrostatographic machine and, moreover, because the coated pipe is subsequently removed from the indexer and placed on a transport pallet for transport to the next processing a subsequent processing station.
Thus, upon completion of the bottom edge wipe process step, the bottom edge solution tank is lowered away from the pipes and the indexer is rotated another 90.degree. to return the pipes to the pallet/pipe transfer station. At this stage, the pipes are lowered back onto the transport pallets, returned to the automated conveyor and transported along the conveyor to a drying and cooling station. As described, this process is repeated for each of three coating layers dip coated onto each hollow pipe for producing a drum-type photoreceptive member.
The above-described dip coating system and process has many disadvantages. The primary disadvantage of this system involves the fact that each step in dip coating a layer of material onto a pipe includes a transfer step wherein the pipes are shifted from the transport pallets on the automated conveyor into each coating booth and subsequently again shifted from each coating booth back to the transport pallets. In fact, it is this very step of transferring each pipe back to the transport pallet that necessitates the bottom edge wipe process at each coating booth for preventing contamination of this coating layer as well as for preventing residual coating material to be deposited on the transfer pallet. Clearly, since this bottom edge wipe process is separately repeated for each layer of the dip coating process, the elimination of this step is desirable and would be greatly advantageous in increasing production throughput, in decreasing overall production facility cost and in ultimately decreasing product cost.
A major disadvantage of the dip coating process system presently in use concerns real estate requirements; that is, in the known system in present use, each dip coating booth must be separately laid out and separated by an independent drying and cooling station for dip coating an individual layer on each workpiece. It is evident that each separate and independent dip coating booth and oven/cooling station requires an incremental addition to physical space. This is not only important in terms of the size requirements of the manufacturing facility, but is also Important in determining the cost of the facility and, necessarily, the ultimate cost of the photoreceptive drums produced therein. This problem is exacerbated by the fact that the entire assembly line facility including each booth and the conveyor system is preferably housed in a class 100 clean room enclosure.
A further disadvantage of the above-described system results from the requirement for separate dip coating booths including separate and independent hardware to yield essentially the same operation at each booth. In the described system, the indexing mechanism provides essentially the same function in each dip coating booth: transporting the pipes from the pallet/pipe transfer station to the dip coating tank; from the dip coating tank to the flash off station; from the flash-off station to the bottom edge wipe station, and finally, from the bottom edge wipe station back to the pallet/pipe transfer station. It would be advantageous to consolidate these repetitive steps into a singular apparatus which could transport a plurality of drums through each dip coating step of the multilayered dip coating process.
An improvement in dip coat processing is an in-line configuration where the workpieces are attached to a carrier pallet to eliminate load/unload steps at each dip coating station.
In another technique for the dip coating of drums, a drum is suspended from a chuck which is mounted on the lower end of a mandrel or carrier pallet. The mandrel is transported by an overhead conveyor from one dip coating tank to another. When a drum reaches a dip coating position over a coating tank, the mandrel is lowered from a home position to immerse most of the drum in a coating liquid retained in a dip coating tank. In plant production lines, photoreceptor drums of several lengths are coated in different coating runs. In a coating many sizes of photoreceptors, it is difficult to maintain an optimal cycle time. Since the pull rate for dipping is usually constant, a short length drum can be coated in less time than a long drum. However, a line that handles multiple length drums must be constructed so that it can also handle dip coating of long drums. This means that for a short drum a significant amount of time is wasted just moving the chuck and mandrel downwardly to where the coating tanks are located. Thus, for example, a short substrate would have to move 250 mm downwardly in order to contact the coating solution. At a lowering speed of 1000 mm/minute, this is 15 seconds of lost time as compared to a long substrate having a length of 500 mm. Again, when the coating cycle has been completed and the substrate must be raised to its home position, another extra 250 mm must be traversed at a time of 15 seconds for a total lost time of 30 seconds. This problem is exacerbated when a coating line must apply a plurality of coats of different materials to each drum at different coating stations. Thus, when a production line is set up for dip coating long drums, such as drums used for double width printing, significant cycle time is lost when the line is subsequently used to coat short drums. More specifically, time is lost because the chuck must be moved a greater distance from the home position to (1) dip a short drum into the coating liquid in the dip coating tank and (2) remove the short drum from the coating liquid in the dip coating tank back to the home position.
While the above-described photoconductive devices are suitable for their intended purposes, there continues to be a need for the development of improved processes and devices which dip coats drums more efficiently.